I'm starting to install GFCI outlets in this house I just bought. I'm
starting with the circuit where the fridge would be. From what I can
tell right now, the fridge is on its own circuit, but I haven't verified
every single outlet; just ones on the same part of wall on both sides of
the wall. The problem is that I have 4 wires total, one green (ground),
one red, and two white wires.

I'm not sure at all how to wire this up. The old outlet had green to
ground, red on the top left, white #1 on the top right, and white #2 on
the bottom right. How would I wire this for a GFCI outlet?